The invention relates to a selective electronic trip unit comprising:
at least one current sensor supplying signals representative of primary currents, PA1 a processing unit comprising means for selective instantaneous tripping connected to the current sensor and supplying a tripping order according to signals supplied by said sensor. PA1 first means for detecting an operating range connected to the current sensor, PA1 second means for detecting repulses producing primary current interruptions between two half-waves of primary alternating current, and PA1 means for analysis, connected to the first and second means for detection, to disable supply of a tripping order if repulses are for primary currents lower than a preset threshold. PA1 a step for measuring a signal representative of a primary current flowing in an electrical conductor to be protected, PA1 a step for comparing the measured signal to a first preset threshold, PA1 a step for determining the start of repulse observation, PA1 a detection phase of predetermined duration to detect the occurrence of signals representative of repulses among the measured signals, and PA1 an analysis step to disable instantaneous tripping if signals representative of repulses occur for primary currents lower than a second preset threshold. PA1 a step for measuring a signal representative of a primary current supplied by a current sensor, PA1 a step for comparing said measured signal to a first preset threshold, PA1 a step for detecting when the measured signal drops to zero or close to zero, PA1 a detection phase of predetermined duration to detect peaks in said measured signal, and PA1 an analysis step to disable instantaneous tripping if the detected peaks are lower than a second preset threshold.
Known electronic trip units generally comprise processing units to perform the thermal or long delay tripping, magnetic or short delay tripping, and instantaneous tripping protections. To achieve selectivity between two circuit breakers connected in series or cascade, it is known to have a high rating in an upline circuit breaker and a low rating in a downline circuit breaker. In this case the instantaneous tripping selectivity is current-based. The instantaneous tripping threshold of the downline circuit breaker is lower than that of the upline circuit breaker.
It is also possible to use a time-based selectivity between two circuit breakers. In this case the trip unit of the upline circuit breaker has a greater instantaneous tripping time delay than the tripping time of the downline circuit breaker trip unit.
These two types of selectivity are not very efficient when short-circuit currents are very high as current thresholds are exceeded in both the circuit breakers and too long a time delay of the trip unit is liable to result in premature wear of the upline circuit breaker contacts. To avoid these drawbacks, selective trip units comprise devices to count beats or opening-closing cycles of the contacts on a short-circuit fault. A trip unit of this kind is notably described in European Patent Application EP-0,128,084.
However, state-of-the-art selective trip units must be used in voluminous and bulky limiting circuit breakers able to withstand a high number of opening-closing cycles and therefore able to guarantee efficient selectivity with respect to the circuit breakers connected downline.
The use of such selective trip units in normal circuit breakers of small dimensions is liable to reduce the endurance of said circuit breakers. This shortcoming is essentially due to the beat counting time which may be several AC current cycles.